1. Field of the Invention
The invention relates to a semiconductor device. In particular, the invention relates to a semiconductor device in which a semiconductor element and a capacitor are both embedded in a resin molding.
2. Description of Related Art
Semiconductor packages (semiconductor devices) in which semiconductor elements that deal with large currents are covered with a resin molding are sometimes used as voltage converters and inverters for driving traction motors of electric motor vehicles (Japanese Patent Application Publication No. 2010-056206 (JP 2010-056206 A), Japanese Patent Application Publication No. 2006-303455 (JP 2006-303455 A), Japanese Patent Application Publication No. 2012-129309 (JP 2012-129309 A) and Japanese Patent Application Publication No. 2003-289129 (JP 2003-289129 A)). Semiconductor devices are sometimes termed semiconductor cards or semiconductor modules. Typically, the aforementioned semiconductor elements are transistors that are used in switching circuits. A resin molding is molded by injecting resin into a mold. In the mold, an assembly made up of a semiconductor element, lead frames, and so on has been disposed. The lead frames made of metal plates are disposed on two opposite surfaces of the resin molding. The semiconductor element within the molding is electrically connected to each lead frame. The lead frames correspond to terminals that are electrically connected to the semiconductor element, that is, correspond to electrode plates. Therefore, the term “lead frame” may be reworded simply as “electrode plate”. Furthermore, for physical connection (joint) between the lead frames and the semiconductor element, a solder material is typically used.
There is known a structure in which a cooler is placed abutting, via an intervening insulator plate, each of lead frames constituting two opposite surfaces of a semiconductor package so as to efficiently cool the semiconductor device. The lead frames provided on the two opposite surfaces of the resin molding also serve as heat radiating plates that diffuse heat from the semiconductor element. Furthermore, the semiconductor element is covered with the resin molding in order to protect the semiconductor element from external dust or moisture and to restrain fatigue degradation of the solder material that joins the semiconductor element and the lead frames. The semiconductor element and the lead frames are different in coefficient of thermal expansion. Therefore, when the semiconductor element produces heat, the semiconductor element and the lead frame that are in contact with the two opposite surfaces of the solder material expand with different coefficients of expansion, so that stress occurs in the solder material. Stress occurs repeatedly in every cycle of heat production, which causes fatigue degradation. A portion of each lead frame is embedded in resin. By covering the semiconductor element and the lead frames with the resin molding, deformation of the semiconductor element and the lead frames is restrained, and stress that occurs in the solder material is restrained, and fatigue degradation of the solder material is restrained.
Sometimes, in a semiconductor package, a capacitor is covered with a molding together with a transistor. The capacitor is connected in parallel with the transistor in order to lessen the surge current to the transistor. In any one of the semiconductor devices disclosed in JP 2010-056206 A, JP 2006-303455 A, JP 2012-129309 A and JP 2003-289129 A, a capacitor and a transistor are covered with a molding.
The transistors that are higher in heat resistance and higher in speed are demanded. Concretely, a SiC (silicon carbide)-based MOS transistor that is more excellent in heat resistance and responsiveness than conventional IGBTs is considered promising. Accordingly, the capacitors are required to have high heat resistance and high responsiveness. As a capacitor excellent in heat resistance and responsiveness, a ceramic capacitor is considered promising. However, the ceramic capacitors have a drawback of being hard and being apt to break easily. Thus, there is a possibility that if a ceramic capacitor is covered with a resin mold together with a transistor, the different thermal expansion coefficients of the resin and the ceramic capacitor cause stress to concentrate in the ceramic capacitor, so that the ceramic capacitor is apt to break easily.